Over the years, Ethernet has become the most commonly used method for local area networking. The IEEE 802.3 group, the originator of the Ethernet standard, has developed an extension to the standard, known as IEEE 802.3af, that defines supplying power over Ethernet cabling. The IEEE 802.3af standard defines a Power over Ethernet (PoE) system that involves delivering power over unshielded twisted-pair wiring from a Power Sourcing Equipment (PSE) to a Powered Device (PD) located at opposite sides of a link. Traditionally, network devices such as IP phones, wireless LAN access points, personal computers and Web cameras have required two connections: one to a LAN and another to a power supply system. The PoE system eliminates the need for additional outlets and wiring to supply power to network devices. Instead, power is supplied over Ethernet cabling used for data transmission.
As defined in the IEEE 802.3af standard, PSE and PD are non-data entities allowing network devices to supply and draw power using the same generic cabling as is used for data transmission. A PSE is the equipment electrically specified at the point of the physical connection to the cabling, that provides the power to a link. A PSE is typically associated with an Ethernet switch, router, hub or other network switching equipment or midspan device. A PD is a device that is either drawing power or requesting power. PDs may be associated with such devices as digital IP telephones, wireless network access points, PDA or notebook computer docking stations, cell phone chargers and HVAC thermostats.
The main functions of the PSE are to search the link for a PD requesting power, optionally classify the PD, supply power to the link if a PD is detected, monitor the power on the link, and disconnect power when it is no longer requested or required. A PD participates in the PD detection procedure by presenting a PoE detection signature defined by the IEEE 802.3af standard.
If the detection signature is valid, the PD has an option of presenting a classification signature to the PSE to indicate how much power it will draw when powered up. Based on the determined class of the PD, the PSE applies the required power to the PD.
The IEEE 802.3af standard describes power distribution over the Ethernet by using the common mode voltage between 2 sets of twisted pairs within the CAT-5 cable. Presently the specification calls for distribution of up to 13 W over 4 of the 8 wires typically contained in the unshielded twisted pair (UTP) CAT-5 cable. However, power distributed over the cable may be increased. Due to safety regulations, it is difficult to increase the voltage sent over the cable. Thus, the current must be increased above the present 350 mA maximum. When the current increases, resistance in the CAT-5 wiring absorbs more energy. This can lead to heating the cable or reducing power being available for consumption at the end of the cable.
The cable resistance can be reduced by using all of the wires in the CAT-5 cable. This should cut the cable resistance in half. If all of these wires are used to deliver power, it becomes extremely important to ensure the connectivity and conductivity of all 8 wires in the cable.
To reduce power loss in the cable and ensure that the cable is capable of carrying the power, it would be desirable to create a measuring mechanism to determine resistance of wires in the cable.
The PoE system treats a pair of wires connected to the same transformer winding as a single conductor. Therefore, there is a need to measure the DC resistance of each of these pairs.
Moreover, the PoE system connects 2 wires together through a transformer's winding. Thus, wires are connected in pairs with the winding forming a DC short circuit between them. If one of these wires has much higher resistance than the other, the transformer will saturate, blocking Ethernet data transmission. Therefore, it would be desirable to determine the individual resistance of each wire in the cable independently of each other.